Island forming a ring (5)
I believe the answer is:
atoll
'island' is the definition.
(thesaurus)
'a ring' is the wordplay.
'ring' becomes 'toll' (tolling is a kind of ringing).
'a'+'toll'='ATOLL'
'forming' acts as a link.
(Other definitions for atoll that I've seen before include "tropical island" , "Ring-shaped coral reef enclosing a lagoon" , "String of islands" , "Bikini, perhaps" , "Coral island(s) surrounding a lagoon" .)